Generally, solar cells are known as devices that convert solar radiation into electrical energy. Typically, solar cells are fabricated on a semiconductor substrate using semiconductor processing techniques to form a p-n junction near a surface of the substrate. Solar radiation incident upon the surface of the substrate creates electron and hole pairs in the bulk of the substrate. The electron and hole pairs migrate to p-doped and n-doped regions in the substrate, thereby generating a voltage differential between the doped regions. The doped regions are coupled to metal contacts on the solar cell to direct an electrical current from the cell to an external circuit coupled thereto. Radiation conversion efficiency is an important characteristic of a solar cell as it is directly related to the solar cell's capability to generate electrical power.
FIG. 1 is an illustration of a cross-sectional view of a typical homogeneous emitter solar cell structure 100. As shown in FIG. 1, a highly doped p+-type silicon emitter 102 is formed on n-type silicon substrate 101. Metal grid lines, such as a metal grid line 104 are formed on the emitter 102. An antireflective coating (“AR”) 103 is deposited on the portion of the emitter 102 between the grid lines. The conventional homogeneous emitter, such as emitter 102, has uniform doping profile underneath and between grid contacts. The active dopant concentration at the surface of the homogeneously doped emitter is generally at least 1020 cm−3 to form an ohmic contact with the grid line and to obtain a high fill factor which is typically defined as a ratio of the actual maximum obtainable power to the product of the open circuit voltage and short circuit current.
High concentration of the active dopants at the surface of the emitter creates high surface recombination velocities. High surface recombination velocity limits open-circuit voltage (Voc) and short-circuit current (Jsc) directly limiting the conversion efficiency of the solar cell.
A selective emitter is used to avoid the limitations caused by the homogeneous emitter. Selective emitters have high dopant concentration beneath grid lines and low dopant concentration between the grid lines. The conventional selective emitter techniques require two or more processing steps to achieve this.
One selective emitter technique starts with the lightly doped Si emitter. Then highly doped silicon paste is selectively applied through a mask to the regions of the lightly doped Si emitter where the grid lines are going to be placed. Then, the grid lines are formed on the highly doped silicon paste regions.
Another selective emitter technique starts with highly doped Si emitter. A hard mask is deposited on the highly doped emitter. The portions of the highly doped Si emitter are etched back through a hard mask to reduce doping in those portions of the Si emitter that are between the grid lines. The grid lines are then deposited on the un-etched highly doped regions of the emitter.
Another selective emitter technique uses at least two separate steps of ion implantation to produce high doping of the emitter underneath the grid lines and low doping of the emitter between the grid lines.
All conventional selective emitter techniques require complicated alignment processing and have generally low throughput. The surface doping achieved with these techniques provides high sheet resistances that are more than 100 Ω/sq. Such high sheet resistances cause a lot of power loss, so that conventional selective emitters require up to 50% more grid lines than the homogeneous emitter. Because the grid metallization typically contains silver, this is a very expensive requirement.